Standard cell sensors or actuators are generally based on flow cytometry and employ one or a combination of electrical impedance sensing, light scattering measurement, and chemical or immunostaining followed by optical sensing.
For differentiation of blood cells by electrical impedance sensing, red blood cells are removed by lysing in order to reduce the blood volume. Lysing is generally done through the use of saponin or surfactants. During the lysing process, the leukocyte cell volume changes depending on cell type, due to the leakage of cytoplasm contents and cell nucleus shrinkage in varying amounts. Fujimoto, Sysmex J. Int. 9 (1999). Thus, normally 2-part (lymphocytes versus granulocytes) or even 3-part (lymphocytes, neutrophils, and other leukocytes) differential can be achieved by simple electrical impedance measurement of particle volume. Hughes-Jones, et al., J. Clin. Patol. 27; 623-625 (1974); Oberjat, et al., J. Lab. Clin. Med. 76; 518 (1970); Vandilla, et al., Proc. Soc. Exp. Biol. Med. 125; 367 (1967); Maeda, et al., Clin. Pathol. 27; 1117-1200 (1979); Maeda, et al., Clin. Pathol. 9; 555-558 (1982). Combining direct current and alternating current impedance, special acidic hemolysis in basophile channel and alkali hemolysis in eosinophil channel, a 5-part leukocyte differential can be achieved. Tatsumi, et al., Sysmex J. Int. 9; 9-20 (1999).
Alternative optical methods are based on light scattering and fluorescence staining of organelles, granules, and nuclei. Generally, low-angle scattered light contains information on cell size and high-angle scattered light can be used to probe internal composition of the cell. To achieve 5-part differential, certain leukocyte populations, such as eosinophils, require special stain to change its scattering characteristics from other granulocytes, and basophils typically need to be counted separately following the differential lysis of other leukocytes. McKenzie, Clinical Laboratory Hematology, Prentice Hall, 2004; Fujimoto, Sysmex J. Int. 9 (1999).
In general, conventional automated cell analyzers are bulky, expensive, and mechanically complex, which restricts their locations to hospitals or central laboratories. Conventional cell analyzers require larger sample volumes and generate more waste than the systems developed using microdevices. Furthermore, for analysis of certain cell types, such as leukocytes, accuracy and speed of counting, differentiation, and/or sorting is important for determining disease state and treatment.